It can be used as a noun as well as adjective.
Meaning : an unconventional or independent minded person.It is actually used to refer to an unbranded animal especially a calf separated from its mother and it is considered as the property of the person who first brands it.So it is used to indicate the person who exhibit unorthodox,independent views and who refuses to conform to established standards of conduct.
Synonyms : individualist, rebel, dissident, dissenter,unconventional,unorthodox,non conformist,recusant
Antonyms : conformist,orthodox,conventional
1)The maverick politician was not popular with his party men.
2)He was too much of a maverick to hold high office.
3)She had no chance of getting promoted because of her maverick behaviour.